Reason to use scoped_refptr here is, all runtime instances share the same root window and root window should be deleted automatically when last runtime instance is released. For example, we launch an application from local path, the main runtime instance as well as the root window are created for the first page(name page A here). It's possible that other pages close page A, under this circumstance,  the root window should still be alive although the  main runtime instance is deleted.